Yersina pestis, the ethiologic agent of plague or Black Death, has in historical times caused devastating pandemics unrivaled by any other infectious disease. Due to its very rapid replication and effective immune evading capacity, Yersinia pestis has recently been recognized as a potential tool for bioterrorism. In particular, an aerosolized delivery of the bacterium could cause a rapid and fulminant pneumonic infection, which subsequently may spread from person to person. Although several vaccines exist, and Yersinia usually is sensitive to streptomycin and tetracycline type antibiotics, the pneumonic form of the disease is difficult to treat and still often results in death. Additional supportive treatments may significantly reduce this lethality, especially in the case of massive exposure of a population to weaponized Yersinia. This grant application focuses on a key component of the molecular machinery by which Yersinia pestis evades the immune system, a highly active protein tyrosine phosphatase (PTPase) termed YopH. In infected hosts, the bacteria multiply in lymph nodes, where they adhere to T and B cells and inject them with YopH. Inside the lymphocytes, YopH efficiently inhibits lymphocyte activation and the development of an immune response. It has been suggested that YopH interferes with early T cell antigen receptor signaling, but the precise mechanisms remain unknown. This application will address the following two Specific Aims. Specific Aim 1 is titled "Molecular targets of YopH in T cell antigen receptor signal transduction". These studies will utilize our familiarity with early T cell antigen receptor-induced signaling events and the molecular mechanisms by which different PTPases regulate these events. We plan to determine the molecular mechanism by which YopH inhibits TCR-induced T cell activation using substrate-trapping technology, proteomics, tryptic peptide mapping, confocal microscopy and functional assays and read-outs for T cell activation. Specific Aim 2 is titled "Development of a YopH-specific inhibitor". In this Specific Aim, we will develop a YopH-specific small molecule inhibitor by first screening the Burnham Institute's chemical library using a colorimetric assay and the BioMek2000 robotic station that was established for this purpose. After two levels of screening and counter-screening with several other PTPases, we anticipate being left with a few inhibitory compounds, which will be evaluated for efficacy in the T cell signal transduction assays established in Specific Aim 1. The results obtained in this study will enable us to better understand the molecular mechanisms by which Yersinia pestis evades the adaptive immune system using the YopH PTPase. This information, together with a specific YopH inhibitor that works in the same T cell activation model, will represent valuable progress towards the goal of designing a drug to combat plague mortality.